Algae have been considered for many years as an excellent candidate as a renewable resource for natural oils. Algae have very high natural oil content, with some species having natural oil content greater than 50% of their total mass. In addition, algae can potentially produce 10 to 100 times more oil per cultivated acre than oil-producing land crops such as soybean, rapeseed, and palm. In addition, algae can be cultivated away from existing croplands on as yet uncultivated, unproductive, and desert lands. In addition, algae can be cultivated on potable or non-potable fresh water or on brackish or salt water. Thus, utilization of algae for production of oil in place of dry land crops may provide many benefits, not the least of which is potential avoidance of climate alteration form greenhouse gas production from fossil fuel usage.
The oils produced by algae can be utilized in a wide variety of applications. For instance, algae are the highest yield feedstock found to date for biodiesel production, lubricants, and the like. Biodiesel, a biodegradable, non-toxic fuel formed from transesterification of any of a variety of vegetable oils or animal fats, has long been considered a viable option to petroleum-based diesel (petrodiesel). Biodiesel can be utilized as formed in unmodified diesel engine vehicles and furnaces, can be easily blended with petrodiesel, and typically produces about 60% less net carbon dioxide emissions than petrodiesel. It is estimated that between 250 and 300 billion gallons of diesel oil is used annually in the United States for transportation fuels and home heating oil. Of this amount, only about 8% comes from renewable resources. Moreover, while relatively few automobiles in the U.S. utilize diesel fuel, the opposite trend exists in Europe, with total diesel consumption in the U.S. from trucks, buses and other transportation estimated to be about 80% of Europe's consumption level.
Between 1978 and 1996, the U.S. National Renewable Energy Laboratory (NREL) examined the possibility of using algae as a biodiesel source. These studies resulted in a collection of approximately 300 different species of algae, both fresh-water and salt-water, and made them available to researchers from around the world. This initial work on algal biodiesel development was curtailed in the mid-90's, due primarily to a drop in crude oil prices and government budget cuts. Interest in biodiesel from algal oils has revived due to both increasing crude oil prices and increasing interest in energy independence from fossil fuels.
While algae present an excellent oil source for a variety of applications including biodiesel production, problems must still be overcome in development of cost-effective, large scale algal production systems. For instance, many mass algal culturing operations require expensive species control techniques to maintain culture purity and the desired large oil recovery volumes. Moreover, harvesting, concentrating and drying microalgae biomass is difficult and costly. In addition, methods used to extract the oils from algae is one of the more costly aspects of the overall process and can in and of itself determine the viability of a production facility.
The most common algal harvest and oil extraction method utilizes mechanical and physical processing systems, usually combined with a solvent extraction process, for instance the well-known Soxhlet extraction process. Such processes are generally quite expensive in order to ensure suitable disruption of the algae cells and facilitate separation of the oils from other plant components. In addition, there are environmental and processing concerns associated with the solvents, usually hexane and benzene, as the process is usually carried out at high temperatures, leading to environmental issues due to solvent volatility.
Another algal extraction method contemplated is a supercritical fluid extraction process, but this method is also quite expensive, requiring pressure vessels and high energy input. Other methods have been considered including enzymatic extraction, osmotic shock extraction, and ultrasonic-assisted extraction, but these methods are expensive and not yet viable for a large scale production process.
What are needed in the art are environmentally friendly, low cost methods for obtaining oils produced from an algal feedstock.